Certain optical fiber waveguides exhibit the property of photosensitivity which provides a practical means for photoinducing permanent refractive index changes in the core of those fibers. Photosensitivity is not restricted to fiber structures: it has also been detected in several types of planar glass structures, including, for example, silica-on-silicon and ion-implanted silica waveguides devices.
The fabrication of optical waveguide devices such as intra-mode retro-reflecting Bragg gratings, mode convertor gratings, and rocking rotators have been achieved. The general approach for making these devices is to photoinduce a refractive index grating in the photosensitive core of the optical waveguide. The grating consists of a periodic modulation of the core's refractive index along the length of the waveguide. The period of the perturbation is chosen to bridge the momentum (propagation constant) mismatch between the two (normally bound) modes that the grating is designed to couple. At the resonant wavelength of the structure, phase-matched, efficient, power exchange between the coupled modes is possible.
There are two basic methods used for photoinducing gratings in photosensitive optical fiber waveguides: either by internal or by external writing. Internal writing is usually a holographic process where the modes to be coupled are launched as coherent bound modes of the waveguide and are allowed to modify, by a two-photon absorption process the refractive index of the waveguide core (i.e. form the hologram). Subsequent launching of one mode "reconstructs" the other. The activation wavelength for writing gratings internally in Germanium-doped high-silica glass is in the visible band (for example, at the 514.5 and 488.0 nm Argon-ion laser wavelengths) with corresponding two-photon energy in the U.V. band. External writing uses UV light directly (for germanium doped high-silica fiber, UV light tuned to, or in the vicinity of, the oxygen vacancy absorption band at 240 nm) incident from the side on the optical waveguide. External writing can be accomplished point-by-point, for mode convertor gratings, or using the holographic interference of two coherent UV beams for Bragg retro-reflectors.
Index gratings were first written in optical fibers using a technique described by K.O. Hill et al and disclosed in U.S. Pat. No. 4,474,427. The process requires launching into the core of a Ge-doped fiber strand light having a wavelength in the visible region. The light is reflected from the end of the fiber. The forward propagating light interferes with the backward propagating light to form a standing wave pattern with a period corresponding to half the wavelength of the writing light. Through a photosensitive effect in the fiber, a refractive index grating with this period is written in the core of the fiber. With this technique, only gratings can be fabricated which reflect light having wavelengths close to the writing light.
An improvement on this technique for writing grating has been disclosed by Glenn et al in U.S. Pat. No. 4,807,950. In that process, the gratings are produced in the fiber by illuminating the fiber from the side with a coherent ultraviolet radiation having 245 nm wavelength. By using a two beam technique, an interference pattern is set up along the length of the fiber. The period of the pattern is controlled by controlling the angle between the interfering beams. Thus index gratings can be written in the fiber which will reflect light at much longer wavelengths.
A further improvement on the above-noted methods for writing gratings in optical fibers is the point-by-point writing technique which is disclosed in U.S. Pat. No. 5,104,209. In this patent, a point-by-point technique for writing gratings in fibers is disclosed in which each index perturbation in the grating is photoinduced individually through a slit-mask.
The principal drawbacks of the grating fabrication technique described in the first patent is that only gratings with a period similar to that of one half the wavelength of the writing light can be made. The second patent discloses a method of writing gratings with a different pitch. However, the technique requires an ultraviolet laser source with a high degree of spatial and temporal coherence. Such laser sources are research lasers that are expensive, have low writing efficiencies and are not suitable for use in a manufacturing environment. Furthermore, the technique does not provide full flexibility in the writing of apodized Bragg reflectors or chirped Bragg reflectors.
The point-by-point writing method is an effective technique for writing the coarse period gratings needed in spatial and polarization mode converters. However, this technique is not practical for writing Bragg gratings. In the case of Bragg gratings, the writing of each index perturbation individually requires high accuracy in the translation of the optical fiber in front of the slit. A more serious drawback, is the serial manner for writing the index perturbations forming the Bragg grating. That writing process needs a very long exposure time to fabricate a single Bragg reflector. U.S. Pat. No. 5,104,209 proposes to overcome this problem by the use of slit-masks to permit the writing of several index perturbations in a single operation.